Transistors are semiconductor devices in which the current flowing between two device regions is controlled or modulated by an applied voltage. Transistors may be categorized as either field effect transistors (FET's) or bipolar junction transistors (BJT's). Bipolar junction transistors are active semiconductor devices formed by a pair of P-N junctions, namely an emitter-base junction and a collector-base junction. An NPN bipolar junction transistor has a thin region of P-type material constituting the base region between two regions of N-type material constituting the emitter and collector regions. A PNP bipolar junction transistor has a thin region of N-type material constituting the base region between two regions of P-type material constituting the emitter and collector regions. The movement of electrical charge carriers that produces electrical current flow between the collector region and the emitter region is controlled by a voltage applied across the emitter-base junction.
Conventional bipolar junction transistors are fabricated with a vertical arrangement of the emitter, base, and collector regions in which these regions have a stacked planar construction formed on a planar surface. As a result, conventional bipolar junction transistors have a relatively large footprint that consumes a significant surface area of the active device layer. The device footprint cannot be reduced because the area of the emitter-base junction cannot be easily scaled. Consequently, the emitter-base junction in planar device designs is limited by the planar surface area.
What is needed, therefore, are semiconductor device structures for bipolar junction transistors and fabrication methods that overcome these and other disadvantages of conventional semiconductor device structures for bipolar junction transistors and methods of manufacturing such semiconductor device structures.